The pelletization of extrudable materials is of considerable importance for many applications. Pellets, unlike ingots or bars, readily flow in measuring and dispensing apparatus and the size of pellet charges can be readily controlled to small tolerances. Moreover, unlike powders, they do not form dust and are not ingested by persons working with them. Thus, they provide a highly convenient form for the packaging, storage and use of many thermoplastic polymers, food products, etc.
Underwater pelletizers are known which employ a rotating disc cutter to cut or sever stranded polymer as it exits from the die plate of an extruder. The cutting is accomplished in a chamber full of circulating water which functions to cool the strand and also to carry away the cut pellets. The prior art disc cutters are of two types: (1) knives which extend radially from a central hub or (2) multiple blades which are attached to spoked hubs.
Attempts to use the prior art disc cutters to pelletize extrudable materials such as fluidic materials which require a relatively long time to solidify have resulted in agglomeration of extruded materials in the pelletizers. The extruded materials often are trapped in the area between the cutting hub and the die face and agglomerates into "trashouts". The extruded materials also agglomerate and wrap around the extended knife blades.
Pelletization of thermoplastic polymers, especially high melt flow thermoplastic polyolefins, have been particularly difficult using prior art underwater pelletizers. This problem is especially eminent in the production of ultra high melt flow and ultra low viscosity adhesive grade buterie-1-ethylene copolymers which contains from about 0.1 to 8 wt % of ethylene which are cracked by a free radical generator. The problem appears to relate to the slow crystallization rate of these polymers which exhibit extreme tackiness in pelletizing. It is known that as the pellets leave the cutting blades, they are very tacky and collide with other pellets to form agglomerates. With a longer residence time, the pellets change to an opaque color, as they complete their crystallization, become dense and lose their tackiness. Excessive turbulence around the trailing edges of the knives also contribute to the agglomeration problem. These agglomerates wrap around the cutting blades and create smears and chunks, plugging the pelletizer chamber, the spin dryer and the area between the die and hub. The extrusion line has to be shut down in order to clean the plugged section resulting in undesirable production interruptions. The agglomerations also result in an excess amount of non-uniform or real formed pellets which may be described by terms such as tailed pellets, long-string pellets, pellet marriages, elbows, dog bones, and pellet trash which are undesirable.
Referring to FIG. 1 and FIG. 2, U.S. Pat. No. 4,621,996, issued Nov. 11, 1986, and assigned to Gala Industries, proposes a conventional underwater plastic pelletizing machine which includes a number of flat cutting blades 21 mounted on a spoked hub 23 on a driven shaft 25 so that the extruded strand of plastic will be cut into a plurality of pellets. The cutting blades 21 proposed project outward, according to the drawing, from the spoked hub 23. There is a gap 29 between the spoked hub and the die face. This cutter design would not process a high melt flow, adhesive grade polymer properly because polymer smears in long strands and wraps around the cutting blades 21 and sharp edges on the hub 23. This is particularly true when the polymer processed has a relatively low viscosity, high tackiness and long crystallization time which make underwater pelletizing very difficult. It is known that excessive turbulence around the trailing edges of the knives also contributed to the wrapping problem.
It is also known that the extruded polymer pellets are entrapped in the gap 29 between the die face 31 and the hub creating smears and chunks, and thus plugging up the pelletizer. It is not practical to use this prior art pelletizer to pelletize adhesive grade ultra high melt flow polybutene-1-ethylene described above because of the frequency of shut downs required for cleaning up the plugged section.
As used herein, a high melt viscosity polymer is a polymer having a melt viscosity 1,000,000 cps or more; and an ultra low melt viscosity polymer is a polymer having a melt viscosity of about 300,000 cps or lower. A polymer with a melt viscosity of about 300,000 cps will have a melt index of approximately 100 dg/min, and is generally regarded as an ultra high melt flow rate polymer with an ultra high melt index. As used herein, the melt viscosity is measured by Brookfield Viscometer using ASTM D2556 at 350 degrees Fahrenheit (.degree. F.), unless otherwise specified e.g. as measured at 275.degree. F. As used herein, the melt flow rates or melt indices are measured by ASTM 1238 condition E at 190.degree. C. and 2.16 kg wt.